Two sentences dog trainers and behavior consultants hear pretty much every week of our lives: “But he was wagging his tail!” and “The bite came out of nowhere!”

A while back, I devoted an episode to the first one, so if you’re still thinking that all tail wags are friendly, go have a listen or a read to the episode called What Do Dogs Say with Their Tails?

This week, it’s the turn of the infamous Bite Out of Nowhere – which, you may not be surprised to hear, almost always comes with previews if you know how to look..

“My Dog Would Never Bite Anybody”

Aggressive behavior is normal. It’s normal for humans, it’s normal for chimpanzees, it’s normal for naked mole rats, and it’s normal for dogs to protect their resources and themselves against perceived threats. Among social animals, most aggressive behavior is “ritualized,” meaning that conflicts are resolved without serious damage being done on either side. You can see a lawsuit as a form of ritualized aggression – it’s nasty, but bloodshed doesn’t generally follow. Among dogs, the classic loud, spectacular dog-park spat in which nothing worse happens than some wet fur and a nick or two is an example of ritualized aggression.

Not only is aggressive behavior normal; it’s a truism that any dog, if pushed far enough, will bite. The real question is what constitutes pushing far enough. The answer varies from dog to dog, and that’s where the “bite threshold model” comes in.

The “Bite Threshold” Model

The eminent dog trainer Jean Donaldson describes the bite threshold in her book The Culture Clash. A dog who perceives a threat experiences stress, and as stressors accumulate, the dog gets closer and closer to his individual breaking point – the “threshold” at which he aggresses.

Here’s an example. Let’s say Dogalini is somewhat uncomfortable with kids. Also, loud noises frighten her. Also, her family doesn’t often have new visitors or take her for walks in public places, and she’s gotten a bit shy around strangers. Also, she’s getting on in years and her hips are often sore with arthritis.

Dogalini deals with her discomfort around kids by going into another room when the grandchildren visit, so everybody in her family thinks of her as “fine” with children. Her hips ache, but nobody’s regularly knocking into them, either. The occasional loud noise makes her dash behind the sofa, but when she emerges a minute later, she doesn’t look the worse for wear. As for those strangers – well, she never sees any, so no one has noticed that she’s now anxious around them.

But what happens when all those things stack up? The grandkids and their parents come over for dinner, along with two other couples they’re friends with. Strangers and kids! Dogalini heads for a quiet corner.

One of the strangers says, “I have a way with dogs,” meaning that he thinks it’s a good idea to follow shy Dogalini into that corner and gaze into her eyes. She cowers and makes herself small; eventually, he gives up. Meanwhile the kids are running around, shouting and tumbling as kids do; Dogalini hides behind the sofa, panting. An hour into the visit, one of the kids takes a header over the back of the sofa and slams into her sore hip.